Hi all,

This seems to be the place where the most knowledgeable people on the GM hybrids hang out, and I was hoping someone might be able to help me with an issue I've been having with my Tahoe. I originally noticed that every once in a while after starting up, the Tahoe would hesitate to move when pressing the gas pedal. This only occurred after startup, and it would go away after a minute or so. It was very troublesome as we live off of a major road and it is very dangerous for the car to not accelerate when needed. As this was an intermittent issue, I spent considerable amount of time trying to narrow down the conditions which lead to this behavior, using information found on the net and a bluetooth OSB adapter with the Torque app on my phone.

What I found was that this issue was mostly repeatable by shutting the engine off with the battery at a low state of charge, either by driving only under battery power alone for the last few minutes of my trip, or by sitting in the car on auto-stop until the engine kicked on to being to recharge. After parking the car under these conditions, the next time I would drive I would experience the hesitation issues. The issue is pronounced if I have the air conditioner running on high.

I utilized the Torque app to monitor the hybrid battery to try and understand at a deeper level what the issue could be. I've put my data in a google sheet here. I have 4 different instances where I let the battery SOC go down, and the last two sheets are instances where the car was hesitating after start up. What I noticed is that as the SOC goes below 50%, the variation in the voltage begins to get higher, reaching greater than 1.5 volts.

I took it to the dealer and they indicated there really isn't a way to verify that the battery isn't performing to expectations since it hasn't thrown any DTC codes. Can anyone verify if this performance is acceptable? Are there any other tests that I can do to check the battery health?

Thanks for any help!

 

The dealer needs to do a better search onbulletins regarding testing the HV battery. What you are describing could be an early warning of a weak cell. The dealer can record all 20 internalvoltmeters inside the HV battery during one of your events described.

 

Wow.

You have a horribly imbalanced battery. I mean horrible. I don't know the internals of the Tahoe battery, or its management, but the numbers suggest that each voltage is reporting the sum of 12 cells. I'm shocked to see voltages in the 12V range with measly 12.3A current.

I don't even understand how it's reporting 50% SoC at those levels. GM must use some creative battery management.

Voltage variations under those loads should be less than 0.3V.

The data in your sheet vigorously indicate your battery is effectively dead. If that were a Toyota product, it would have thrown a code a long time ago.

 

Hillbilly,

The dealer had a Tech 2 scanner that I pushed him to use. On the tech 2 I didn't see an option for each voltage, but only the max and min voltage. I'll see if I can get him to dig into the options further to monitor and graph each cell voltage during a hesitation event. Will there be anything in particular we should look for during this exercise?
Looking at the voltage data i collected during these hesitation events it seems that something is wrong, but it isn't clear to me exactly what that is. Which is pretty much the explanation I'm getting from the dealer. Since they couldn't pin it down, they just quoted me $1200 for a tune up, fuel injection service, top engine cleaning, and replace spark plugs and wires with the hopes that will fix it. I don't doubt that the engine could slightly benefit from a tune up, but after making them explain their reasoning for their suggested work I don't feel confident that it will address the specific issues I'm concerned with.

Keith,

As far as I understand, the Tahoe hybrid battery normally operates between ~65 and 45% SOC to not stress the battery and extend it's life. On the high end when the battery is at 60-65% SOC it typically shows a voltage between 16-18V. But when the battery is drained to less than 50% SOC the voltage drops and I've seen it go down into the 11V range on some cells, while others always stay above 13V. I don't know if this makes sense, but that is why I'm here hoping to get some other opinions!

 

It does not make sense at all unless I'm really missing something.

nominal 300V battery should be 250 cells

You have 20 voltage blocks, which would be 12.5 cells per block. not possible.

Based on this presumed picture of the internals:

https://i.ytimg.com/vi/nw1QzfoheaI/maxresdefault.jpg

those are 6 cell Prius modules. 2X in series forming each voltage block would put the pack nominal at 288V. The initial voltages you report all jibe with that.

That level of voltage disparity should never happen. My experience is completely with Toyota systems, but those modules are toyota modules. Period. They should never report voltages below 13.2V. At a minimum, you shouldn't see significant voltage disparities under those light loads in a healthy battery. 0.3V max; 0.2V more typical.

Issues may also be your reader. It could be getting/reporting bogus data.

If your numbers are correct, and IF the dealer equipment shows similar min/max V variations, you have a **** battery.

 

So based on the battery test results from the idaho national laboratory, the Tahoe hybrid battery has 240 cells (modules), split between the 20 voltage blocks i'm measuring means that each block has 12 cells. It also states that the nominal cell voltage is 1.2V, times 240 cells gives a nominal pack voltage of 288, which makes sense and matches their description. During the HPPC test they performed, they noted the max cell charge voltage of 1.49V and minimum cell discharge voltage of 1V . Multiplying these by the block of 12 cells gives block max voltage of 17.88V and a minimum block voltage of 12V.

These values combined with the fact that the dealership scanner showed similar max and min voltages makes me feel that my data isn't completely wrong. Also, the dealer did notice on his Tech 2 scanner that the minimum voltage was 11.8 with a 12.8V maximum at the same point.

This is the way I understand it, but I've only been looking at this stuff for the past few months so I could be wrong. But I do feel confident that I'm on the right track, but I'm just lost as to how to convince the dealer that the issue is battery related and will not be solved with a $1200 tune up without actually paying for the tune up...

 

Okay. It's basically identical to a Lexus GS450h battery pack. I have one on my bench right now.

First, the 45-65% SoC limit places additional restrictions on voltage limits. Furthermore, those limits are also current dependent. Today, I saw low 13s at 60% SoC, but it was during a full throttle acceleration with 120A load.

The currents you had in your capture are pitiful. There is no way a healthy battery at 45% SoC would every permit such low block voltages.

Again, if the numbers you have recorded are accurate, your battery is absolutely, positively bad. No question. Not only that, the battery management is not working or horribly horribly badly designed. I don't know the failure criteria before a code is reported, but I wouldn't rule out the that reporting mechanism is inoperative.

If you have a means of escalating to a higher authority, I recommend you pursue it immediately. Anyone with specific training in the system should be able to identify that there is a major problem.

 

If there is a real bad cell, just 1 of the 240 that minimum voltage on the scanner will drop into single digits.


That said a max to min difference of 1.5 volts with less than a 30 amp load means at least one is getting weak.


As for fuel injectors... a $10 bottle of Techron. Cleaning the electric throttle body is a good idea. Plugs and super conducting wires are good too.


Pack voltage should never drop below 200 when in EV up to 20kW Those 20 volt-meters in the pack I think are able to be read in Snap Shot mode on Tech 2. Lots of special functions and ways to record data.

 

with 240 cells, there's no way it will ever get anywhere near 200V. I would expect trips below 264V to be RARE, momentary and under HEAVY load.

 

Under real high loads and lower SOC they will. The Panasonic NiMh chemistry will be damaged beyond any cycling repair if any one cell goes below 0.7 volts and a high load is run through it. Remember these things can pull 40 kW in short bursts. During peak Re-Gen voltage can creep up on 400. SOC and temperature have a big influence.


That said the delta on min to max should always be less than 1 volt.
So his pack has at least one weak/bad cell.


As for convincing a novice dealer that this battery has a problem... and it does... a 2008 is out of warranty and a pack is about $2400 plus install.